Molecular orbitals unsaturated hydrocarbons

Aromaticity is usually described in MO terminology. Cyclic structures that have a particularly stable arrangement of occupied 7t molecular orbitals are called aromatic. A simple expression of the relationship between an MO description of stmcture and aromaticity is known as the Hiickel rule. It is derived from Huckel molecular orbital (HMO) theory and states that planar monocyclic completely conjugated hydrocarbons will be aromatic when the ring contains 4n + 2 n electrons. HMO calculations assign the n-orbital energies of the cyclic unsaturated systems of ring size 3-9 as shown in Fig. 9.1. (See Chapter 1, Section 1.4, p. 31, to review HMO theory.)... 

Bond [294] used comparisons between homogeneously and heterogeneously catalysed interconversions of unsaturated hydrocarbons to deduce that the reactive state of an adsorbed hydrocarbon may reasonably be assumed to be a jr-complex (see Sect. 3.2, p. 22). On this assumption, a molecular orbital model appropriate to a face-centred cubic metal was developed. By considering the direction of emergence and degree of occupation of the metal atomic orbitals at the (100), (110) and (111) faces, assuming that the atomic orbitals on the surface keep the same orientation as in the bulk metal, which may not be valid [295], he concluded that the (111) planes were least suited to the adsorption requirements of... 

To further illuminate the LCAO variational process, we will carry out the steps outlined above for a specific example. To keep things simple (and conceptual), we consider a flavor of molecular orbital theory developed in the 1930s by Erich Hiickel to explain some of the unique properties of unsaturated and aromatic hydrocarbons (Hiickel 1931 for historical... 

Hydrocarbon monomers containing n bonds form n complexes with transition metals. Figure 2.1 shows the overlap of the molecular orbitals involved in the formation of a n bond between the simplest unsaturated hydrocarbon monomer, such as alkene, and transition metal [3],... 

Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules. I. See also, R. Pariser and R. Parr, J. Chem. Phys., 21, 767 (1953). A Semi-Empirical Theory of the Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules. II. J. A. Pople, Trans. Faraday Soc., 49, 1375 (1953). Electron Interactions in Unsaturated Hydrocarbons. J. A. Pople, J. Phys. Chem., 61, 6 (1957). Application of Self-Consistent Molecular Orbital Methods to ti Electrons. 

Hydrocarbon Based Polymers. The substitution of one hydrogen atom in the -CH2-CH2- unit by some short carbon chains induces subtle modifications in the electronic structure (molecular orbitals) of the polymers. Though these modifications cannot be easily evidenced on the XPS carbon Is core level spectra, it appears that the XPS valence band structures are much more sensitive to these substitutions and that they become unique and readable fingerprints of the polymers (1, 22). We will not speak here of the Cls shake-up data that were revealed useful to distinguish between saturated and unsaturated bonds (this field with various applications was recently reviewed (23)). 

A special class of cyclic unsaturated hydrocarbons is known as the aromatic hydrocarbons. The simplest of these is benzene (C6H6), which has a planar ring structure, as shown in Fig. 22.11(a). In the localized electron model of the bonding in benzene, resonance structures of the type shown in Fig. 22.11(b) are used to account for the known equivalence of all the carbon-carbon bonds. But as we discussed in Section 14.5, the best description of the benzene molecule assumes that sp2 hybrid orbitals on each carbon are used to form the C—C and C—H a bonds, while the remaining 2p orbital on each carbon is used to form 77 molecular orbitals. The delocalization of these 1r electrons is usually indicated by a circle inside the ring [Fig. 22.11(c)]. 

The irradiation of aromatic compounds results in considerably lower yields of radiolysis products than does irradiation of aliphatic compounds of similar molecular weight and functional group composition. This has been attributed to effectiveness of the delocalized 7t-orbitals in accommodating excitation energy without permitting the molecule to dissociate. Nevertheless, some radiolysis does occur. Benzene is known to yield biphenyl, phenylcyclohexadiene, and a polymeric material of average composition (C6H7) which behaves as if it were an unsaturated hydrocarbon. Dimerization and polymer formation are also characteristic of the radiation... 

Three recent developments in the molecular orbital theory of electronic structure are particularly worthy of note. These are first, the successful analysis and quantitative interpretation of the spectra of unsaturated hydrocarbons and their simple derivatives 7 secondly, the development of a sound descriptive theory of the structures of electron-deficient compounds and thirdly, the elucidation of the principles underlying the structures and stabilities of inorganic metal complexes. These three topics are largely independent and each could occupy in itself a review of this length. However, the recent advances in the theory of inorganic metal complexes have been admirably covered in a recent review by... 

Moffitt and Ballhausen,41 and the application of the molecular orbital theory to electron-deficient compounds, particularly the hydrides of boron, by Eberhardt, Crawford, and Lipscomb has been fully described by these authors in their original papers.14 The present review will, therefore, be devoted entirely to those developments in the molecular orbital theory which have been associated with its application to the electronic spectra of unsaturated hydrocarbons. These developments form in themselves a relatively coherent story, the main lines of which are now clear, and it seems a suitable moment at which to put the history of the subject into perspective. Before doing this, however, it will be convenient to outline the general premises of the molecular orbital theory. 

J. A. Pople, Trans. Faraday Soc., 49, 1375 (1953). Electron Interactions in Unsaturated Hydrocarbons. J. A. Pople, J. Phys. Chem., 61, 6 (1957). Application of Self-Consistent Molecular Orbital Methods to tt Electrons. 

In the olefin complex the carbon-carbon double bond is believed to be perpendicular to one of the valency directions of the transition metal. The theoretical treatment of Tr-bonding by Chatt and Duncanson (77) furnishes the basic idea underlying the molecular orbital treatment for a variety of complexes between transition elements and unsaturated hydrocarbons. 

A necessary theoretical condition for the occurrence of striking excess ground-state stability is the possibiliy of cyclic delocalization of tt electrons in a molecular closed-shell ground-state configuration constructed from a set of cyclically overlapping pn atomic orbitals. Huckel s famous rule s) states that only those planar monocyclic conjugated unsaturated hydrocarbons containing (4 m + 2) 7T electrons fulfill this theoretical condition. 

Space does not unfortunately permit more than a mention of the free-electron molecular orbital theory and its application to the spectra of unsaturated hydrocarbons and heteromolecules. The very recent calculations of Ham and Ruedenberg on unsaturated hydrocarbons cover a more extensive range than the LCAO calculations of Pariser, with which they agree very well. It seems fair, however, to say that in spite of brilliant exploratory work in this field the free-electron theory in its present form still lacks foundations as secure as those which have now been provided for the LCAO theory. A particular difficulty in the free-electron molecular orbital theory is the proper inclusion of electron repulsion various ways have been devised of introducing it into the theory but the validity of these expedients still rests on goodwill rather than on rigor. Nevertheless the free-electron theory, in the hands of Platt and Kuhn, has already pointed the way to a sound theory of the spectra of linear and branched systems, and there seems little doubt that the next few years will witness advances in the theory of d)re spectra - as important as those which have already occurred in the theory of simpler molecules. 

In molecules, the conjugation of unsaturated systems, i.e., possessing molecular orbitals built up from Ti-atomic orbitals (2p-orbitals in hydrocarbons benzene), is, in general, a planarity factor (e.g., butadiene). Nevertheless, steric hindering can oppose this tendency to make a planar system (e.g., biphenyl, cyclooctatetraen ). The same phenomena are encountered in crystals. 

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